One of the advantages brought about by the introduction of semiconductor technology is an increased useful life of semiconductor devices over their prior art vacuum tube equivalents. It is, however, well known to those familiar with the semiconductor art that semiconductor devices can be rapidly destroyed by exposure of their active surfaces to atmospheric conditions while such devices are in operation. Quality semiconductor devices are, consequently, routinely shielded from environmental conditions by an application of passivation layers over active surfaces, and, in many cases, by hermetically sealing device packages which ultimately house the devices.
Hermetically sealed housings, envelopes or packages of articles which contain complex and relatively expensive integrated circuits are routinely leak-tested by sophisticated instruments. The tests involved the use of radioactive gases as a vehicle to detect leaks in the packages. A relatively high cost of the involved instruments is justifiable because of the inherent value and the functional important of such integrated circuits. On the other hand, simple, low cost transistors or diodes are not routinely subjected to such leak tests involving radioactive gases and sophisticated instrumentation.
Because of an ever-present requirement for price competitiveness of low priced devices, a leak test tends to be looked upon as disproportionately raising the cost of the product without significantly enhancing its statistical quality. Consequently, leak tests, even gross leak tests, as a routine step frequently tend to be bypassed in the production of low cost devices. As a result, any lot of such low cost devices may include a certain number of devices which tend to have a comparatively shorter useful life before failure.
In the manufacture of axially leaded diodes which are housed in hermetically sealed glass sleeves and which are intended to be used in electrical circuits of apparatus with high lifetime requirements, a statistically acceptable sampling plan is employed. A portion of all manufactured diodes is subjected to a gross leak test by which inadequately sealed diode housings are detected. To conduct the test, the diodes are placed in a pan and submersed in a liquid dye which fluoresces when irradiated by a black light source. The pan is loaded into a pressure chamber and subjected to a pressure of 1000 psi for approximately one hour.
Thereafter the dye is rinsed from exposed portions of the diodes, and the diodes are visually inspected under a microscope with a typical 20.times. magnification while being illuminated with a black light source. When viewed under these conditions, defectively sealed diodes appear illuminated by small remaining dye portions which have penetrated into the diode housing during the pressurization period and have remained there after the rinsing operation. Typical quality specifications may call for complete inspection by the above visual test of all manufactured diodes when more than eight diodes are detected as being defective in a typical lot of six thousand diodes.
Of course, optical inspection of small devices, such as the diodes, is cumbersome and time consuming and, hence, expensive. Such expense, even when incurred only on statistical samples, adds to the cost of the final product. A low cost leak test is therefore desirable, since the routine electrical tests to which all of the diodes are subjected prior to shipment do not detect a defective hermetic seal of a housing unless a particular device fails already during testing because of a defective seal. Unfortunately, the effects of a defective seal typically show up only after a prolonged operation, at which time the diodes have found their way into an intricate piece of apparatus, and a failure at that time can be catastrophic.